How to smooth a curve from an already existing model?
You can create brand new curves in blender, and then convert them to those tubes with needed resolution. How to do it, there is a lot of tutorials on youtube. Try to search something like "create pipes with curves in blender".
As for your existing meshes, you can convert them to curves. But segments you would have to handle anyway.
If you want to go this way, you have to get the single edge loop from your tube: Select your tube; go to Edit mode (Tab); switch to edge select mode (Ctrl+Tab, 2). Then select one entire longitudal edge (Alt + RMB on that longitudinal edge); and kill all others (Ctr+I - invert selection; X - delete Edges).
It will give you the wire-like mesh one vertex wide. Here it's good to switch to vertex selection mode (Ctrl+Tab, 1). You can use proportional editing (O) to edit it, use Smooth (Ctrl+V, Smooth), (W, Loop tools / Space), (W, Subdivide smooth) and any other tools you find useful. When you done with the mesh, return to the Object mode (Tab), and convert the mesh to curve (Alt + C, mesh to curve). And how to deal with curves you already know from the tutorials about pipes, that I suggested before :-)